The present invention relates to a method and apparatus for separating particles from a fluid suspension by filtration. In particular the present invention relates to an acoustically driven method and apparatus for filtering fine particles in a filtering medium having an average pore size substantially larger than the diameters of the particles being filtered. As used herein, the term "particles" encompasses solids, immiscible liquid droplets, gas bubbles, and other types of discrete matter that might be suspended or entrained in a fluid.
Filtration of fluid suspensions containing submillimeter-size particles is of fundamental importance in many chemical and biological processing applications. Conventional separation approaches include physical screening techniques (mechanical sieves, beds of filtration media, or porous membranes in which the fluid passes through pores smaller than the size of the solid particles being collected), gravity-driven methods that accomplish separation based on the difference in densities of the particles and the host fluid (centrifugal and settling techniques), and procedures that involve external fields (such as electrical or magnetic) to enhance the quality or rate of separation based on specific system properties.
Filtration of fine solid particles is often very difficult, however, due to the strong interactions between the solids and their host liquid. In the case of conventional screening methods, high pressure drops or slow processing rates often result from the plugging of membranes or the blocking of pores by the particles. Furthermore, back-flushing of a membrane to regenerate the filtering medium often is difficult due to strong interactions between the particles and the filter substrate itself Moreover, suspensions of immiscible liquid droplets and gas bubbles typically cannot be filtered, as these types of particles are capable of distortion or splitting to pass through the pores in the filtering medium.
In the past few decades, methods based on the use of ultrasonic standing wave fields have been developed for separation of particles from liquid streams without reliance on filtration media. These methods exploit the density and/or compressibility difference between suspended particles and the host liquid to yield sharp, highly efficient separation of particles using resonant acoustic fields In the case where a one-dimensional sound field is used, the particles are organized into thin parallel bands separated by a one-half wavelength spacing. The particles then are separated from their host fluid by placing closely spaced physical barriers between the bands of particles, transporting particles in the opposite direction of the flowing host liquid by using pseudo-standing waves, or relying on gravity to settle the swarms of particles organized by the acoustic field. These particle-harvesting techniques, however, can be problematic for practical applications. The first two approaches are difficult to achieve mechanically because of the typical small separation distances involved, and the third technique can be hindered by slow sedimentation rates.
Acoustic fields also have been applied in membrane or sieve filtration processes. In these applications, however, intense ultrasonic fields are used to create vibrations in the filtering medium (or in the cake formed above the medium) for the limited purpose of preventing or reducing clogging. Essentially, these ultrasonic fields are applied to free particles from the filtering medium, not to enhance the medium's filtering efficiency.
The concept of combining a porous filtering medium with an external field to enhance particle separation has proven useful in dielectrophoresis and high-gradient magnetic separation. These methods are limited, however, to particles having certain electrical or magnetic properties and therefore are not suitable for a wide variety of applications.
The present invention is intended to utilize the imposition of an acoustic field to increase the efficiency of a porous filtering medium.
In particular, the present invention is intended to enable a porous medium to collect particles up to three orders of magnitude smaller than its average pore size.
The present invention also is intended to provide an acoustically driven enhanced filtration system that does not rely upon the electrical or magnetic properties of the particles being separated and is capable of successfully filtering liquid droplets and gas bubbles as well as solid particles.
In addition, the present invention is intended to provide a system for harvesting particles and regenerating the filtering medium by alternately activating and deactivating an acoustic field imposed on a porous filtering medium.
Additional advantages of the present invention will be set forth in part in the description that follows, and in part will be obvious from that description or can be learned by practice of the invention. The advantages of the invention can be realized and obtained by the method and apparatus particularly pointed out in the appended claims.